A conventional stacking apparatus 10 is illustrated in FIG. 1. The stacking apparatus 10 is configured for use adjacent to a rotary die cut machine 12 which cuts blanks (not illustrated) from sheets of material, for example, corrugated paperboard. The stacking apparatus 10 includes a receiving or “layboy” section 14 that receives the blanks from the die cut machine 12 and discharges them onto a transfer conveyor 16. The transfer conveyor 16 carries the blanks to an inclined main conveyor 18, and the blanks travel along the main conveyor 18 to its downstream end 20 where they are discharged into a hopper 22.
After the blanks are discharged from the downstream end 20 of the main conveyor 18, they impact against a backstop 24 and fall either a) directly onto a discharge conveyor 28 or b) onto elevating fingers 26 which controllably lower stacks of the blanks onto the discharge conveyor 28. As the stack 30 on the elevating fingers 26 grows, the elevating fingers 26 drop, either continuously or periodically, so that the sheets leaving the main conveyor 18 are always falling approximately the same distance from the downstream end 20 onto the elevating fingers 26 or onto the partial stack 30 on the discharge conveyor 28. In other embodiments, the sheets may fall on a fixed height platform or conveyor, and the downstream end 20 of the main conveyor 18 may rise to stay a relatively fixed distance above the top of the growing stack 30.
When the stack 30 has reached a desired height, the elevating fingers 26 lower the stack 30 to a level even with the discharge conveyor 28, if elevating fingers 26 are used, and the discharge conveyor 28 moves the finished stack 30 away from the stacking apparatus 10. When the stack 30 has been transferred from the elevating fingers (or when the stack has moved away from the location beneath the hopper 22 if the stack was formed directly on the discharge conveyor 28), the elevating fingers 26 rise toward the hopper 22 for receiving additional sheets from the downstream end 20 of the main conveyor 18.
The rotary die cut machine 12 operates substantially continuously, and sheets of material therefore continue to traverse the stacking apparatus 10 and reach the hopper 22 even when a finished stack is being removed from the discharge conveyor 28 and/or when the elevating fingers 26 are lowering the stack 30 toward the discharge conveyor 28. During the time that the stack 30 is being removed from beneath the hopper 22, accumulator shelves 32 are extended to receive sheets as they leave the downstream end 20 of the main conveyor 18. When a finished stack has been removed from beneath the hopper 22 and the elevating fingers 26 are back in position for receiving additional sheets, the accumulator shelves 32 retract and drop the sheets that have accumulated thereon onto the elevating fingers 26 or onto the discharge conveyor 28. Additional sheets exiting the downstream end 20 of the stacking apparatus 10 fall onto the stack, and the process repeats until the stack on the elevating fingers 26 or the discharge conveyor 28 reaches a desired height.
It is common to include a tamping device in the hopper 22. Such a tamping device repeatedly presses in against the stack on the accumulator shelves 32—either from one or both sides or from the front and/or back, to align or square the small stack on the accumulator shelves 32. It is often desirable to finish squaring or tamping the stack on the accumulator shelves 32 before withdrawing the accumulator shelves 32 and dropping the small stack onto the elevating fingers 26 or the discharge conveyor 28.
Modern rotary die cut machines and stackers operate at increasingly high speeds, and the number of sheets transported per minute is thus increasing. To maintain a high throughput, it is desirable to keep the rotary die cut machine and the stacker operating continuously. However, with present stacker designs, it is difficult or impossible to finish tamping a small stack of sheets on the accumulator shelves and drop that small stack from the accumulator before the next sheets start to fall from the end of the main conveyor. This is particularly true when the stackers employ a blowing device to cause the sheets exiting the discharge end of the main conveyor to fall faster than they would under the force of gravity alone, particularly in the case of large sheets that tend to float on a cushion of air as they drop. In such devices, it is difficult or impossible to consistently time accumulator operation so that a laterally extendable accumulator shelf can be inserted into a falling stack of sheets without either damaging the edges of the sheets or possibly causing a jam.